I. Field of the Invention
The present invention relates to a method of fabricating diffraction gratings and a photomask for use in the same. More specifically, the invention relates to a method of fabricating diffraction gratings by the contact exposure method utilizing electromagnetic waves and, particularly, ultraviolet rays and SOR rays, the diffraction grating being suited for the production of optical elements that contain a diffraction grating therein, such as a distributed feedback (DFB) laser, a distributed Bragg reflection (DBR) laser, an optical amplifier, a spectroscope, and the like. The invention further relates to a photomask used therefor.
II. Description of the Related Art
Diffraction gratings have heretofore been fabricated by the so-called holographic exposure method according to which a laser beam is branched and is then synthesized again as disclosed, for example, in Japanese Patent Laid-Open No. 148050/1974. In particular, the diffraction grating having a uniform grating period is fabricated by the above-mentioned method.
The diffraction gratings include a diffraction grating having a uniform grating period and a .lambda./4-shifted grating in which the phase of diffraction grating is inverted. These gratings are fabricated by a positive and negative photoresist method based upon the holographic exposure method, or by a phase image projection method. The positive and negative photoresist method has been discussed in IEICE TECHNICAL REPORT, OQE 85-11, April, 1985, pp. 69-76, and the phase image projection method has been discussed in IEICE TECHNICAL REPORT, OQE 85-60, July, 1985, pp. 57-64.
The above related arts have problems as described below. First, with the holographic exposure method, visibility (bright/dark ratio) of fringe decreases due to mechanical vibration and sway of the air, making it difficult to obtain a diffraction grating maintaining good reproduceability. When it is desired to expand the area of exposure, in particular, the exposure must be continued for an extended period of time. Therefore, degradation of visibility becomes a serious problem. Second, it is difficult to fabricate a diffraction grating having a nonuniform grating phase, such as a phase-shifted diffraction grating in which positions of ruggedness on the grating are partly changed and a chapted diffraction grating in which the period of ruggedness is changed at a relatively short period.
With the positive and negative photoresist method, furthermore, the reproduceability is deteriorated since a mid-layer must be made present to prevent the resists from being mixed by each other. Further, with the phase image projection method which involves an interference pattern caused by multi-reflection due to phase image plates that are closely contacted to the substrate,
it is difficult to obtain a diffraction grating that produces little noise.